N-(3,3-dimethylbutyl)-ATM has a sweetening potency, on the weight basis, of at least 50 times that of aspartame and about 10,000 times that of sucrose (table sugar) so that it can constitute a high intensity sweetener.
Since sweetening agents are mainly employed in foods for human consumption, they must be prepared using a method which can provide a highly purified product substantially free from impurities or decomposition products. Furthermore, in the case of a sweetening substance which tends to be decomposed relatively easily, like N-(3,3-dimethylbutyl)-APM, some countermeasures are required against the decomposition thereof after forwarded as a product.
The already known crystals of N-(3,3-dimethylbutyl)-APM are described in WO95/30689 with reference to the IR spectrum data. The present inventors have confirmed that these crystals are monohydrate crystals as a result of the X-ray crystal structure analysis, and that they show the specific peaks of diffracted X-rays at angles of diffraction of at least 6.0°, 24.8°, 8.2° and 16.5° when measured by a powder X-ray diffractometer using CuKα radiation. The present inventors have decided to call these crystals “A-type crystal” for convenience' sake.
Meanwhile, a preparation process of N-(3,3-dimethylbutyl)-APM is also described in U.S. Pat. No. 5,278,862, wherein high purity (97% by HPLC) N-(3,3-dimethylbutyl)-APM is obtained by crystallization using methanol and water as the crystallization solvent.
And, the present inventors have followed Example 1 of the said U.S. Pat. No. 5,278,862. As the results, although they have confirmed the reproducibility of the data on purity (98% by HPLC), they could not confirm the formation of A-type crystals. In greater detail, the resulting crystals showed, as wet crystals, the specific peaks of diffracted X-rays at angles of diffraction (2θ, CuKα rays) of at least 5.10, 21.10, 21.3° and 8.3°. The powder X-ray diffraction pattern at this time will be given in FIG. 1. These crystals will hereinafter be called “B-type crystal”.
Furthermore, the B-type crystals obtained by following Example 1 of the said U.S. Pat. No. 5,728,862, when dried, gave crystals exhibiting the specific peaks of diffracted X-rays at angles of diffraction (2θ, CuKα rays) of at least 5.6°, 8.4°, 17.1° and 18.8°. The powder X-ray diffraction pattern at this time will be given in FIG. 2. As a result of measurement by Karl Fisher's method, the water content of these crystals were found to be 0.6 wt. %. These crystals will hereinafter be called “G-type crystal”.
The resulting G-type crystals and A-type crystals of N-(3,3-dimethylbutyl)-APM were tested at 70° C., concerning their stability. As a result, after the lapse of 160 hours, the remaining ratio of the N-(3,3-dimethylbutyl)-APM in the form of the G-type crystals was 34 wt. %, while that in the form of the A-type crystals was 94 wt. %, suggesting that N-(3,3-dimethylbutyl)-APM is more stable in the form of A-type crystals. The relationship between the storage time and the remaining ratio of N-(3,3-dimethylbutyl)-APM in this test will be shown below in Table 1.
TABLE 1Stability test at 70° C.Time elapsed (hrs)052160CrystalRemainingRemainingRemainingtyperatio (wt. %)ratio (wt. %)ratio (wt. %)A959594G968734
As described above, it has been found that according to Example 1 of U.S. Pat. No. 5,728,862, G-type crystals of N-(3,3-dimethylbutyl)-APM which are inferior to A-type crystals in stability, are obtained.
(Disclosure of the Invention)
As has been described above, a process for stably preparing A-type crystals excellent in stability, of N-(3,3-dimethylbutyl)-APM at a low cost, has not yet been established in the existing state of art.
Therefore, it is an object of the present invention to provide a process for stably and conveniently preparing highly stable A-type crystals of N-(3,3-dimethylbutyl)-APM, which is a high intensity sweetener.
With a view to attaining the above-described object, the present inventors have carried out an extensive and intensive investigation. As a result, it has been found that, upon crystallization of N-(3,3-dimethylbutyl)-APM from a crystallization solvent consisting of water singly or a mixed solvent of water/alcohol, A-type crystals can be obtained stably as wet crystals by controlling the nucleation temperature, and the type of the crystals to be precipitated can be controlled to be A-type by using A-type crystals as seed crystals; and that dry A-type crystals can be obtained by drying these A-type crystals to have a water content of 3 to 6 wt. % (inclusive of the water of crystallization). Based on these findings, the present invention has been completed. It should be noted that the term “controlling the nucleation temperature” as used herein means “controlling so as to generate nucleation at 30° C. or greater”.
Accordingly, the present invention relates, in a first aspect, to a crystallization method of N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine methyl ester crystals exhibiting the specific peaks of diffracted X-rays at angles of diffraction (2θ, CuKα rays) of at least 6.0°, 24.8 °, 8°, 8.2° and 16.5°, which comprises using water alone or a mixture of water and a lower alcohol as the crystallization solvent and controlling the nucleation temperature at 30° C. or greater, and in a second aspect, to a crystallization method of N-[N-(3,3-dimethylbutyl)-L-α-aspartyl)-L-phenylalanine methyl ester crystals, which comprises, using water alone or a mixture of water and a lower alcohol as the crystallization solvent and using, as the seed crystals, N-[N-(3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine methyl ester crystals exhibiting the specific peaks of diffracted X-rays at angles of diffraction (2θ, CuKα rays) of at least 6.0°, 24.8°, 8.2° and 16.5°, whereby the same type of crystals as the seed crystals are preferentially precipitated.
In the first place, the first-mentioned crystallization method will be described below.
Examples of the lower alcohol usable as a crystallization solvent according to the present crystallization method which belongs to a nucleation-point controlling method, include methanol, ethanol, isopropanol, and the like. Among them, methanol is more preferred because it can be supplied industrially at a low cost.
Although there are no particular limitations imposed on the methanol content in the crystallization system, the methanol content in the solution to be subjected to crystallization is preferably 15 wt. % or less because an excessively high methanol content disturbs smooth crystallization.
A-type crystals can be obtained by nucleation at 30° C. or greater. However, since high crystallization temperatures allow the decomposition of N-(3,3-dimethylbutyl)-APM to proceed, it is, in practice, preferred to generate nucleation at 30 to 65° C., more preferably 40 to 50° C. Concreate methods of such nucleation within the above-described range can be carried out, for example, (a) by maintaining a supersaturated solution of N-(3,3-dimethylbutyl)-APM at a temperature within the above-described range until nucleation occurs, (b) by cooling a saturated or supersaturated solution of N-(3,3-dimethylbutyl)-APM having a temperature of 30° C. or greater and generating nucleation before the temperature is lowered to 30° C., or the like. In such methods of nucleation, application of a mechanical impact such as supersonic or the like from the outside is effective for early nucleation. In short, it is easy for those who are skilled in the art to control the nucleation temperature by using, in appropriate combinations, factors such as concentration of N-(3,3-dimethylbutyl)-APM before beginning of crystallization, temperature, time, cooling rate, mechanical impact and the like.
When crystallization is started in the crystallization solvent system using water alone or a mixture of water/methanol, A-type crystals can be obtained, as has been described above, by controlling the nucleation temperature to be 30° C. or greater. By generating nucleation crystallization at a temperature lower than the above, however, A-type crystals are not formed but B-type crystals are formed as wet crystals. Such nucleation at such a lower temperature must therefore be avoided.
The present crystallization method can be conducted in accordance with the ordinary manners employed in the art as needed, except that the above-described solvent is employed for crystallization and the nucleation temperature is controlled to be 30° C. or greater.
It is needless to say that the present crystallization method can be effected in any one of crystallization operation manners such as batch crystallization, continuous crystallization, agitation crystallization, static crystallization, and the like.
It is well known that in any one of the above-described crystallization operation manners, the solubility of a substance to be crystallized is preferably lowered by using cooling crystallization (cooling operation) in combination in order to heighten the yield of crystals.
In the present crystallization method, it is of course possible to use cooling operation in combination. In greater detail, once A-type crystals are formed by adjusting the nucleation temperature to 30° C. or greater, the increased parts or the increment of the crystals resulting even after cooling of the crystallization system, is also A-type crystals because the already-formed A-type crystals serve as seed crystals. In the present crystallization method, no particular limitations are imposed on until what temperature the solution is cooled before separation of the crystals by filtration.
There are no particular limitations imposed on the apparatus for obtaining dry A-type crystals having a water content of 3 to 6 wt. % by drying the wet A-type crystals of N-(3,3-dimethylbutyl)-APM. A through-flow dryer, fluidized dryer, vacuum dryer, spray dryer, flash drying, pneumatic conveying dryer, or the like can be used freely.
In the next place, description will be made of the second-mentioned crystallization method.
Concerning crystallization, seeding crystallization means, in general, a method of crystallizing preferentially the same kind of crystals as the crystals seeded, by seeding with the crystals in a solution to be subjected to crystallization.
Examples of the lower alcohol usable as a crystallization solvent according to the present crystallization method which belongs to a seeding crystallization method, include methanol, ethanol, isopropanol, and the like. Among them, methanol is more preferred because it can be supplied industrially at a low cost. This is the same as what has been deseribed in connection with the first-mentioned crystallization method.
Although no particular limitations are imposed on the content of the lower alcohol in the solution to be subjected to crystallization, a content of 35 wt. % or less in the solution to be subjected to crystallization is preferred because too high alcohol concentrations disturb smooth precipitation of crystals. The alcohol concentration can be made higher according to this second-mentioned method compared with the first-mentioned crystallization method, because the addition of A-type crystals as seed crystals from the outside of the system makes it possible to omit adjustment of the nucleation temperature at 30° C. or greater.
According to the present seeding crystallization method, although no particular limitations are imposed on the temperature at which A-type crystals are added as seed crystals insofar as the solution is supersaturated at this temperature, preferred is 20° C. or greater, more preferably 25° C. or greater from the viewpoint of crystallizing operation easiness.
Although no particular limitations are imposed on the amount of A-type crystals to be added as seed crystals, preferred is 0.2 to 30% relative to the weight of the initial solute in the solution to be subjected to crystallization, because an excessively large amount deteriorates efficiency.
Although no particular limitations are imposed on the form of A-type crystals to be added as seed crystals, a solid or slurry is preferably added. Also recommended is use, as seed crystals, of the remaining slurry as such in a continuous crystallization.
The present crystallization method can be conducted in accordance with the ordinary manners as employed in the seeding crystallization method except for the above-described crystallization solvent and seed crystals.
It is needless to say that the present crystallization method can be effected in any one of crystallization operation manners such as batch crystallization, continuous crystallization, agitation crystallization, static crystallization, and the like.
In any one of the above-described crystallization operation manners, the solubility of a substance to be crystallized is preferably lowered by using cooling crystallization (cooling operation) in combination in order to heighten the yield of crystals, this being the same as has been described above in connection with the first-mentioned crystallization method.
Also, according to the present crystallization method, it is of course possible to use cooling operation in combination. No particular limitations are imposed on until what temperature the solution is cooled before separation of the crystals by filtration.
There are no particular limitations imposed on the apparatus for obtaining dry A-type crystals having a water content of 3 to 6 wt. % by drying the wet A-type crystals of N-(3,3-dimethylbutyl)-APM. As has been described above in connection with the first-mentioned crystallization method, a through-flow dryer, fluidized dryer, vacuum dryer, spray dryer, flash dryer, pneumatic conveying dryer or the like can be widely used.